1. Field of the Invention
The present invention relates to a quantum cascade laser using intersubband transition in a quantum well structure.
2. Related Background Art
Light in a mid-infrared wavelength range (for example, wavelength of 5 to 30 μm) is an important wavelength range in the field of spectroanalysis. As a high-performance semiconductor light source in this wavelength range, recently, a quantum cascade laser (QCL) has gained attention (regarding the quantum cascade laser, refer to, for example, Patent documents 1 to 5 and Non-patent documents 1 and 2 listed below).
The quantum cascade laser is a monopolar type laser element which generates light by means of electron transition between subbands by using a level structure formed by subbands in a semiconductor quantum well structure, and high efficiency and high output operation can be realized by multistage cascade coupling of quantum well light emitting layers that are formed by quantum well structures and become an active region. This cascade coupling of quantum well light emitting layers is realized by using an electron injection layer for injecting electrons into the emission upper level and alternately laminating quantum well light emitting layers and injection layers.
Patent document 1: Japanese Patent Application Laid-Open No. H08-279647
Patent document 2: Japanese Patent Application Laid-Open No. 2006-032691
Patent document 3: Japanese Patent Application Laid-Open No. 2004-119814
Patent document 4: US Patent Publication No. 5457709
Patent document 5: US Patent Publication No. 5509025
Non-patent document 1: R. Kohler et al., “Terahertz semiconductor-heterostructure laser,” Nature Vol. 417 (2002) pp. 156-159
Non-patent document 2: Benjamin S. Williams et al., “Terahertz quantum-cascade laser at λ=100 μm using metal waveguide for mode confinement,” Appl. Phys. Lett. Vol. 83 (2003) pp. 2124-2126